A thermographic material (hereinafter referred to also as a recording material) for forming an image using a thermal head has advantages such as simplicity of a recording apparatus, no developing process, no noise, maintenance-free and low cost, and is applied in various fields such as printers and facsimile devices.
A conventional thermographic material has the advantages described above, but has problems that undesirable coloring occurs on accidental heating after recording, since only heating causes coloring reaction and that recording is easy to corrupt and low in reliability. Therefore, a thermographic material capable of fixing a formed image has been eagerly sought.
As a typical thermographic material capable of fixing a formed image, a diazo thermographic material is well known, but the diazo thermographic material has the problem that a diazonium compound used is likely to give a background fog due to its high reactivity and results in such a background fog as maximum color density after long-term storage due to poor thermal stability.
Recently, background fog has been reduced by separating the diazonium compound from a coupler using a microcapsule, but the reduction is not still satisfactory, and a more stable thermographic material has been sought.
In Japanese Patent O.P.I. Publication Nos. 1-129247, 1-143252, 3-19252 and 3-1983 is disclosed a thermographic material comprising a microcapsule, in which a photolytic free radical generating agent and a leuco dye are incorporated, with a reducing agent present outside the microcapsule. In this thermographic material, it is possible to mix the reducing agent with the photolytic free radical generating agent and leuco dye by applying heat not to form a dye from the leuco dye, and to entirely expose the heated material to form a dye at portions which have been not heated. However, this type of thermographic material, containing an oxidation developing leuco dye forming a dye on reaction with a photolytic free radical generating agent, has the problems that fog occurs, since the material is subjected to room light or sun light during storage or the photolytic free radical generating agent is gradually decomposed during long term storage. A more stable thermographic material easy to handle has been sought.
The above thermographic material is not colored at heated portions and colored at non-heated portions. This is a thermographic material giving a reversed image different from a conventional thermographic material, and therefore, the usage is limited.
In Japanese Patent Publication No. 43-29407 is disclosed a method of decoloring a dye by imagewise exposure to form an image, but this method has so far not been applied to thermographic materials.
In Japanese Patent O.P.I. Publication Nos. 2-190386, 2-190385 and 2-190383 is disclosed a thermographic material containing an organic boron compound salt of a cationic dye such as a cyanine dye or a xanthene dye, wherein the material is exposed to light which the salt absorbs to decolor the dye and form an image. In this material, a recording image is obtained by mixing, at heated portions, a compound decomposing an organic boron compound with the organic boron compound, whereby the organic boron compound is decomposed to a compound which does not decolor by light exposure, and then by exposing to light to decolor non-heated portions.
However, this thermographic material has various problems, since dyes used or light wavelength used for decoloring the dyes are limited. Such cationic dyes have the problems in that the formed image is poor in stability or light fastness. The cationic dyes are easily decolored by room light, and therefore, there is a problem that a thermographic material using such dyes are difficult to handle under room light. It is necessary to mix a compound decomposing an organic boron compound with the organic boron compound by heating and then decompose the organic boron compound, however, instantaneous heating due to a thermal head is difficult to completely decompose the organic boron compound. Further, such dyes used, which are generally salts, are low in organic solvent solubility and expensive.